1. Field of the Invention
The present invention relates to a semiconductor output circuit, and more particularly, to a semiconductor output circuit which controls power supply to a load.
2. Description of the Related Art
As a semiconductor output circuit using a source follower output transistor for supplying power to a load, for example, there is a circuit disclosed in Japanese Unexamined Patent Application Publication No. Hei 06-188710, which is equivalent to U.S. Pat. No. 5,352,932 (hereinafter, referred to as Tihanyi). The above-mentioned semiconductor output circuit is basically structured by coupling as a source follower an output transistor between a power supply line and an output terminal being coupled with a load, and controlling a conductive state/nonconductive state of the output transistor to control power supply to the load.
In particular, the output transistor is shifted from the conductive state to the nonconductive state by coupling a depletion transistor between the gate and the source of the output transistor.
However, the inventor has noticed the following problem inherent in the output transistor. In the output circuit disclosed in Tihanyi, the source and the substrate terminal (back gate) of the depletion transistor are commonly coupled. In the case where the output transistor is in the conductive state, the depletion transistor is required to be in the nonconductive state, but in some cases, is in the conductive state.
In other words, when the output transistor is in the conductive state, the gate of the depletion transistor is supplied with a ground potential, whereas a source potential of the output transistor, which is nearly equal to a power supply potential, appears on the source thereof. As a result, the voltage applied between the gate and the source of the depletion transistor is in the state exceeding a so-called cut-off voltage, and accordingly, the depletion transistor enters the nonconductive state.
In the output circuit disclosed in Tihanyi, because the conductive state/nonconductive state of the depletion transistor is controlled using the power supply voltage (for example, battery voltage), a device having a higher breakdown voltage is required. A drain current (leak current) with respect to the voltage applied between the gate and the source is compared between a depletion transistor having a higher breakdown voltage and a depletion transistor having a lower breakdown voltage. As a result, it is revealed that in the depletion transistor having a higher breakdown voltage, compared with the depletion transistor having a lower breakdown voltage, the drain current (leak current) hovers at high level in the state of having a value larger by one digit or more (does not drop) and is not completely cut off (does not enter the nonconductive state) even when the voltage applied between the gate and the source is reduced.